How can I access view3d.select_circle radius value?
Operator Properties Last

The window manager has some useful methods re looking up operators and their properties.  Of particular use is WindowManager.operator_properties_last(...)

If we feed in the operator, in this case "view3d.select_circle"

&gt;&gt;&gt; sc = C.window_manager.operator_properties_last("view3d.select_circle")
&gt;&gt;&gt; sc.radius
273


it will return the last used properties of the operator.

If the operator has not been used (or does not exist)  sc will be None

This is not read only. Changing the value will set it for the next run of the operator

&gt;&gt;&gt; sc.radius = 40


Defaults

Possibly worth mentioning too, to get the default value use

&gt;&gt;&gt; rna = bpy.ops.view3d.select_circle.get_rna_type()
&gt;&gt;&gt; rna.properties['radius'].default
25


as it does seem that although the class name is as expected

&gt;&gt;&gt; bpy.ops.view3d.select_circle.idname()
'VIEW3D_OT_select_circle'


after a change in API (around 2.79 IIRC) not all classes are accessible via bpy.types

&gt;&gt;&gt; getattr(bpy.types, 'VIEW3D_OT_select_circle', None) is None
True